Many devices use flexible circuit substrates. Integrated circuits, or chips, mount on the flexible circuit substrate and use the traces in the flexible circuit substrate like a typical circuit substrate similar to a printed circuit board. The flexible circuit substrate generally consists of polymers and other relatively carbon rich materials. In some instances, the carbon of the flexible circuit substrate can become fuel for an arc event, charring, thermal event, etc.
If an electrical short occurs in a chip mounted on the flexible circuit substrate, such as between the negative voltage supply (Vss) and a high voltage positive supply (Vpp), excessive heat results from the excessive current flow between Vpp and Vss due to the low electrical resistance short within the chip. The short within the chip can be created by different factors, including a defective part, electrical over-stress, etc. The high temperature carbonizes the flexible material, and carbon being electrically conductive, this forms another electrically conductive, carbon path outside of chip between the supply traces such as Vpp and Vss or any other nearby trace that becomes involved, such as traces that become shorted to Vpp or Vss, with carbonized flexible material. This further increases the current flow through the lower resistance area, in turn generating more heat. More current produces more heat which carbonizes even more material and the process becomes self-propagating as long as sufficient fuel for generating electrically conductive carbon, such as the flexible circuit substrate, and current from power supplies remain available.
The arc ‘tracks’ along the current path, carbonizing material along the way. The carbonized material, as in the case of polyimide (Kapton™ is a brand name of a polyimide flexible circuit material), may also release hydrogen that readily ignites and consumes more of the flex material until the entire Vss and Vpp signal planes are carbonized. This problem has resulted in certain substrate materials being banned for use in certain industries such as in the aircraft industry. However, using flexible circuit substrates allows for tighter packaging and smaller devices, as well as typically lower costs.